It has been known that ultrafiltration membranes can be produced by wet processes from a variety of resins including polysulfone, polyether sulfone, acetyl cellulose, cellulose, etc. However, ultrafiltration membranes obtained from hydrophobic resins such as polysulfone and polyether sulfone suffer problems originating from their hydrophobicity; that is, when they are used to treat highly hydrophobic aqueous solutions such as hydrophobic aqueous protein solutions, oil emulsions, milk, citrous fruit juice, blood plasma and serum, since such feeds are adsorbed on the membrane surface not only is the membrane fouled but also the quantity of permeating water is decreased with time. Membranes made of cellulose are highly hyrophilic and free from the problems encountered in the hydrophobic membranes, but they have a disadvantage of low resistance to acids and alkalis. Furthermore, it has been pointed out that cellulose is very low in resistance to bacterial degradation.
Membranes made of polyvinyl alcohol-based resins have also been known [see, for example, JP-A-49-53955 and JP-A-52-l23385 (the term "JP-A" as used hereinafter means an "unexamined published Japanese patent application")]. In order to enable membrane formation from polyvinyl alcohol which is a water-soluble polymer and to impart water resistance to the membrane, the proposed methods comprise adding an acetalizing agent (e.g., formaldehyde or a dialdehyde such as glutaraldehyde) and a crosslinking agent to an aqueous solution of polyvinyl alcohol and heating the resulting water-soluble polymeric aqueous solution so as to allow the acetalizing reaction of polyvinyl alcohol to proceed for membrane formation.
Disadvantages of ultrafiltration membranes in common use today which are made of polysulfone or cellulose based resins are summarized as follows:
(1) polysulfone membranes very likely adsorb hydrophobic feeds; and PA1 (2) cellulose membranes have very low resistance to bacterial degradation.
Since the conventional processes of forming ultrafiltration membranes from polyvinyl alcohol-based resins are carried out in an aqueous system and rely upon homogeneous reaction in an aqueous solution, a pore forming agent must be added in order to convert the resin into a substantially porous body such as an ultrafiltration membrane. Furthermore, temperature control for ensuring uniform acetalization is difficult to achieve and the reactivity by the scale of the membrane is not uniform.